In general, electronic equipment cause frequently occurring electromagnetic interference to the environment and to the operation of the equipment. Electromagnetic interference is often caused by regular activity associated with the operation or power supply of the equipment. Interference can be removed by means of frequency domain or time domain signal processing. However, a useful signal and an interfering signal cannot be distinguished from the frequency domain interference cancellation and some of the useful signals are therefore also removed during interference cancellation. For reducing periodic interference, the time domain signal processing employs, for example, coherent averaging where more than one signal sequences are co-phased and summed together for determining the average form of a single signal sequence. The solution can be improved by relating a sampling frequency to the occurrence frequency of a periodic signal by observing the minor changes in the occurrence frequency too. Such a solution is described in the publication Periodic Interference Rejection Using Coherent Sampling and Waveform Estimation, Heinonen, P., Saramaki, T., Malmivuo, J., Neuvo, Y., IEEE Transactions on Circuits and Systems, p. 438-446, vol. Cas-31, no. 5, May 1984, which is incorporated herein by reference.
A similar result is obtained also in the time domain signal processing, if a learning filter is used that learns the form of the interference and reduces the interference from the signal to be processed. Such a solution is described in the publication A Learning Filter for Removing Noise Interference, Furno, G., S., Tompkins, W., J., IEEE Transactions on Biomedical Engineering, Vol. BME-30, no. 4, April 1983, which is incorporated herein by reference.
However, the problem with these solutions is that an interfering signal and a useful signal cannot be accurately distinguished from one another particularly in such cases in which the amplitude of a periodic interfering signal varies or when the useful signal occasionally arrives at the same frequency band together with the interfering signal. Consequently, the interference cancellation is not successful or the interference cancellation also eliminates a significant part of the useful signal.